I did say I didn't know much about this stuff didn't I?
Well that sucks. Big world, small world, it doesn't matter, I get the same problem. I have no issues at all with Vanilla Terraria with the biggest world. I have to imagine if someone with a PC worse than mine can make and join their own world, I should be able to with a better PC using tapi as well.
As my example goes before. My cousin can make the biggest world and join his own world on the tapi server no problem, but I can't join his. I can make my own large world just fine, (biggest, smallest, it doesn't matter) but I can't join it. The error pops up when I try to join, so I don't see why any world size would be an issue with that for me, but not my cousin who has less ram than I do, he's using the same mods as me, and.
This is the error I get with mods on upon trying to join a new world:
http://imgur.com/Y7qosfo
This is the error I get with no mods when trying to join a new world:
http://imgur.com/LzU9ztY